Presbyopia and cataract are two of the most common disorder to beset human vision, especially for the aged. Presbyopia is the loss of the ability of the eye to change focus for near vision. This change is associated with a loss in the ability of the crystalline lens of the eye to change shape. The onset of presbyopia is typically around 40 to 50 years of age. When presbyopia manifests in a person, they will no longer have the ability to attain clear vision for reading or seeing objects up close. This is currently most commonly corrected by the prescribing of reading glasses, bifocal, multifocal or aspheric progressive spectacles and contact lenses. Cataract is a disorder of the eye characterized by a loss of optical clarity of the crystalline lens. The individual with cataract will gradually lose vision in the affected eye. The current method for treating cataract is by removal of the crystalline lens (or its contents) and replacement of the lens with an intraocular lens (IOL) to restore correct distance focus. However, with conventional IOL implantation following cataract surgery, near vision is loss.
To overcome the problem of loss of near vision in presbyopia and post-cataract surgery, and to restore true accommodation (the ability of the eye to change continuous focus) some technologies have become available recently including accommodating IOL (AIOL) such as Crystalens, or Humanoptic 1CU. Many others are currently in design and development stages. These include two-element AIOL (e.g. Synchrony by Visiogen) and other AIOLs (e.g. Nulens). A more natural strategy for restoring accommodation in the presbyope would involve, not the use of optical/mechanical devices such as those mentioned above, but the refilling of the crystalline lens with a soft material. This is a preferred option as the origin of presbyopia stems from a loss of softness of the content of the crystalline lens. Hence, a lens content replacement approach would be the most direct method for restoring accommodation. There are a number of ways by which this could be accomplished. The most direct method (sometimes called “phaco ersatz”; after Parel) involve the injection of a soft gel into the capsule of the crystalline lens to replace the (removed) hardened lens content of the presbyopic eye. Another method (after Nishi) involves the implantation of a bag-like device into the capsule and then fill the bag-like device with a soft gel. Finally, a pre-formed and pre-shaped lens (after Fine) which can be thermally distorted to facilitate introduction into the lens capsule, and its original shape re-established by thermal ‘plastic memory’ may be used.
For all of these methods mentioned above (which we hereafter call “crystalline lens prosthetics” methods), an optimum visual outcome can only be achieved if good knowledge of the shape of the natural crystalline lens as well as the most preferred optimum shape of the refilled lens is available.
It is an objective of the present invention to provide a method by which the surface shape (both anterior and posterior) of the natural crystalline lens and the shape of a crystalline lens prosthetic can be measured, described and used for the design and manufacture of optimum crystalline lens prosthetics.
Numerous analytical and finite element (FE) mechanical models of the human crystalline lens have been developed to simulate changes in lens shape during accommodation. Analytical models have been used to describe the accommodative mechanism in the human eye (Koretz and Handelman 1982) and to investigate the effects of lens elastic anisotropy on accommodation (Koretz and Handelman 1983). FE models have been used to demonstrate that Helmholtzian mechanism of accommodation is most likely for the young lens (Burd, Judge and Flavell 1999), to show that the 29 year old lens is more effective in accommodating than the 45 year old lens (Burd, Judge and Cross 2002), to compare Coleman and Helmholtzian accommodation theories (Martin, Guthoff, Terwee and Schmitz 2005), to estimate the external force acting on the lens during accommodation (Hermans, Dubbelman, van der Heijde and Heethaar 2006) and to show that the maximum zonular tension decreases with age and is the most likely cause for the decrease in accommodative amplitude with age (Abolmaali, Schachar and Le 2007). More recently FE models have been used to analyze the relationship between lens stiffness and accommodative amplitude (Weeber, van der Hiejde 2007) and to determine the change in accommodative force with age (Hermans, Dubbelman, van der Heijde and Heethar 2007). FE models provide valuable information about accommodation and presbyopia. The quality of the models depends on the geometric information used to develop them. Therefore accurate geometric representation of the human crystalline lens is a critical issue for FE modeling, especially at the equatorial regions where the forces are applied.
Burd et al. (2002) and Martin et al. (2005) used geometric information recorded by Brown (1973) to develop models for lenses aged 11, 29 and 45 and therefore their studies are limited to these three ages. Hermans et al. (2006) developed their model using lens shape obtained from Scheimpflug imaging. The images contain only the central portion of the anterior and posterior surfaces of the lens. Missing regions were modeled using two conic functions. Abolmaali et al. (2007) developed their model using information from published MRI images. Their model was not age dependent. Weeber et al (2007) used geometrical information based on in-vivo measurements (Dubbelman, van der Heijde & Weeber 2005; Strenk, Semmlow, Strenk, Munoz, Gronlund-Jacob & DeMarco 1999).
We have found that FE models should account for age dependency of the lens shape and should be based on measurements of the lens shape when no stresses are applied. The isolated ex-vivo lens is not subjected to any active external forces and can therefore serve as the basis for a geometric model that can be used in finite element modeling (FEM) studies.
The human crystalline lens is composed of two aspherical surfaces, which have been modeled with a number of mathematical functions. The earliest eye model represents the lens as two spherical surfaces. The shape has been progressively described as hyperbolic (Howcraft and Parker 1977) parabolic (Koretz, Handelman and Brown 1984) fourth order polynomial (Strenk, Strenk, Semmlow and DeMarco 2004) and conic functions (Dubbelman, van der Hiejde 2001, Rosen, Denham, Fernandez, Borja, Ho, Manns, Parel & Augusteyn 2006). While these models present a good approximation of the human lens, they were developed for optical modeling and therefore mostly focus on the central 4 to 5 mm of the lens, but not for providing much information about the equatorial region. Kasprzak (2000) approximated the whole profile of the human lens using a hyperbolic cosine function. This model is based on published values of radius of curvature and asphericity and focuses on the central optical zone of the lens. While this model has been evaluated against hyperbolic, parabolic and elliptic approximations, it has not been compared to an actual lens shape and therefore, the validity of the equatorial regions of this model is not known. Furthermore this approximation is a relatively complex expression and may not be easy to implement in FE models.